[libc++] [libc++abi] Use C++20 standard.

[curdeius]

Summary

This change is needed to use char8_t when building libc++.
Using the same standard in libc++abi for coherence.

See https://reviews.llvm.org/D91517.

[curdeius]

I see at http://lab.llvm.org:8011/#/builders?tags=%2Blibcxx that there is at least one builder, http://lab.llvm.org:8011/#/builders/128, using gcc 5, that doesn't have -std=c++2a/20 flag. So using CXX_STANDARD_REQUIRED ON will fail there.
Should I set it to OFF?
Otherwise, only gcc 8+ accepts -std=c++2a...

[curdeius]

In order to support older compilers (not accepting -std=c++2a, i.e. gcc < 8 and clang < 5), I think we should use CXX_STANDARD_REQUIRED OFF, so that it will fall back to c++17.

[ldionne]

In D91691#2408062, @curdeius wrote:

In order to support older compilers (not accepting -std=c++2a, i.e. gcc < 8 and clang < 5), I think we should use CXX_STANDARD_REQUIRED OFF, so that it will fall back to c++17.

Okay, let's do that until we actually update our compiler requirements. I have a draft email I need to send, I'll try to do that sooner than later. Thanks for following up, Marek.

[curdeius]

• Make C++ standard non-required, so that compilers that do not accept -std=c++2a are pleased (gcc < 8).

[ldionne]

LGTM once CI passes. The failures looked flaky to me, I restarted the jobs.

[curdeius]

Thanks Louis! The tests that fail seem indeed flaky (C++03: both failures in libcxx/test/libcxx/modules). Especially that the first time (before I changed required standard to OFF) everything passed just fine (and without restarts).
How can I restart a job?

[ldionne]

In D91691#2408780, @curdeius wrote:

Thanks Louis! The tests that fail seem indeed flaky (C++03: both failures in libcxx/test/libcxx/modules). Especially that the first time (before I changed required standard to OFF) everything passed just fine (and without restarts).
How can I restart a job?

You might need to be an admin in Buildkite.

I believe these jobs are flaky because they use modules, and there's something broken with module caches. @EricWF and I have been seeing these failures on and off for a while. It'd kind of disturbing.

libcxx/CMakeLists.txt

 # FIXME(EricWF): See the FIXME on LIBCXX_ENABLE_PEDANTIC.
 # Remove the -pedantic flag and -Wno-pedantic and -pedantic-errors
 # so they don't get transformed into -Wno and -errors respectively.
 remove_flags(-Wno-pedantic -pedantic-errors -pedantic)
 
 # Required flags ==============================================================
 function(cxx_add_basic_build_flags target)
 
-  # Require C++17 for all targets. C++17 is needed to use aligned allocation
-  # in the dylib.
+  # Require C++20 for all targets. C++17 is needed to use aligned allocation
+  # in the dylib. C++20 is needed to use char8_t.
   set_target_properties(${target} PROPERTIES
-    CXX_STANDARD 17
-    CXX_STANDARD_REQUIRED YES
+    CXX_STANDARD 20
+    CXX_STANDARD_REQUIRED NO
     CXX_EXTENSIONS NO)
 
   # When building the dylib, don't warn for unavailable aligned allocation
   # functions based on the deployment target -- they are always available
   # because they are provided by the dylib itself with the excepton of z/OS.
   if (ZOS)
     target_add_compile_flags_if_supported(${target} PRIVATE -fno-aligned-allocation)
   else()

libcxxabi/src/CMakeLists.txt

if (LIBCXXABI_ENABLE_SHARED)
   if (TARGET pstl::ParallelSTL)
     target_link_libraries(cxxabi_shared PUBLIC pstl::ParallelSTL)
   endif()
   set_target_properties(cxxabi_shared
                         PROPERTIES
                           CXX_EXTENSIONS
                             OFF
                           CXX_STANDARD
-                            17
+                            20
                           CXX_STANDARD_REQUIRED
-                            ON
+                            OFF
                           COMPILE_FLAGS
                             "${LIBCXXABI_COMPILE_FLAGS}"
                           LINK_FLAGS
                             "${LIBCXXABI_LINK_FLAGS}"
                           OUTPUT_NAME
                             "c++abi"
                           SOVERSION
                             "1"
if (LIBCXXABI_ENABLE_STATIC)
   if (TARGET pstl::ParallelSTL)
     target_link_libraries(cxxabi_static PUBLIC pstl::ParallelSTL)
   endif()
   set_target_properties(cxxabi_static
                         PROPERTIES
                           CXX_EXTENSIONS
                             OFF
                           CXX_STANDARD
-                            17
+                            20
                           CXX_STANDARD_REQUIRED
-                            ON
+                            OFF
                           COMPILE_FLAGS
                             "${LIBCXXABI_COMPILE_FLAGS}"
                           LINK_FLAGS
                             "${LIBCXXABI_LINK_FLAGS}"
                           OUTPUT_NAME
                             "c++abi")
 
   if(LIBCXXABI_ENABLE_PIC)